Fixing Flutter Is Nothing New

Investigating violent oscillations that led to structural failure

By Peter Katz

MAINTAINING STRUCTURAL INTEGRITY IN THE AIR. Within a six-month period, two Lockheed Electra airliners (one operated by Braniff Airways) were felled due to a phenomenon investigators termed “propeller-whirl flutter,” which could tear an aircraft apart in 30 seconds.

In April, the NTSB advised the FAA to ground all Zodiac CH 601XL S-LSA and E-LSA until the FAA determines they have adequate protection from aerodynamic flutter, which occurs when airplane structures vibrate back and forth in increasingly violent oscillations, eventually reaching a point where the structure breaks apart. According to the NTSB, since February 2006, four U.S. and two overseas accidents involving CH 601XLs could be attributed to aerodynamic flutter causing in-flight structural failure. The FAA responded that it has no plans to ground the fleet but will review the Zodiac’s design. The manufacturer, Zenair, is fully cooperating with the agency; its preliminary findings confirm that the airplane isn’t prone to flutter. It stressed the importance of verifying that all control surfaces are properly fastened and secured, free of play, that control stops are present and properly installed, and that control cables are properly tensioned. Zenair also reminded pilots to operate the aircraft within their design limits.

The NTSB noted that the CH 601XL relies on control-cable tensioning to add stiffness to the aileron system, provide a higher natural-resonant frequency of the aileron/wing combination and raise the airspeed at which flutter may occur. Aircraft in the normal, utility, aerobatic and commuter categories (which are FAA-certified, unlike the Zodiac) typically use a combination of structural stiffness and flight controls that are balanced through the use of added mass (typically a weight) for protection against flutter. The NTSB says this provides more direct protection against flutter; it notes that some LSA use mass-balanced flight controls.

Government certification is no guarantee that flutter won’t be a problem. Over the years, the NTSB has reviewed flutter-related accidents involving familiar makes and models. The original C-141 military transport had early issues with severe tail flutter, as did the F-15 jet fighter. F-16s with wing pods mounted in certain positions also experienced wing flutter.

We’re approaching the 50th anniversary of the September 29, 1959, accident of a Braniff Airways Lockheed Electra four-engine turboprop in Buffalo, Texas. The left wing failed, and the aircraft crashed, killing all 34 on board. Flight 542 was a scheduled flight between Houston, Texas and New York City, with stops at Dallas and Washington, D.C. It departed from Houston for the 41-minute trip to Dallas at 10:37 p.m. At 11:07 p.m., the flight crew radioed Braniff with a message for maintenance. Two minutes later, ground witnesses saw a fire in the sky and heard loud noises. The wreckage was spread over a path that was 13,900 feet long. One recovered item was the flight engineer’s log sheet, filled at 11 p.m. At that time, the airplane was at 15,000 feet at 275 knots, with engine and airfoil anti-icing systems off and normal engine instrument indications. Braniff had taken delivery of the Electra just 11 days earlier; its total time was just 132 hours and 33 minutes. As days became weeks, the investigation didn’t provide answers.

On March 17, 1960, another Electra crashed, near Tell City, Ind; there were no survivors. Northwest Orient Flight 710 had been cruising at 18,000 feet in clear weather. It had departed Minneapolis-St. Paul en route to Miami and was in contact with Indianapolis Center at the time of the in-flight structural failure. At 3:15 p.m., witnesses saw the right wing separate. After this second tragedy, it became clear that an in-depth investigation of the Electra design needed to be done quickly.

Tests began on a one-eighth-scale model of the Electra at NASA’s Langley Research Center in Virginia, where a new wind tunnel had just been calibrated. Engineers found that at certain engine, propeller and airspeeds, the gyroscopic torque of the engine-propeller combination set up a wobbling motion with a frequency of three cycles per second. This number of cycles was the same as the natural flutter frequency of the wings. The oscillations could build up and cause structural failure within 30 seconds. They called the phenomenon “propeller-whirl flutter.” Thenceforth, all Electra engine mounts were strengthened to dampen the unwanted forces before they could turn into destructive wing flutter.

On March 7, 2005, a de Havilland DHC-2 turboprop flew out of Talkeetna, Alaska, with an ATP-rated pilot and three passengers. The pilot was approaching Mt. McKinley from the northeast at 11,000 feet MSL when the DHC-2 began shaking violently. Thinking it was an engine problem, the pilot shut it down, but the shaking continued. When the pilot slowed to 80 mph, the shaking stopped. He restarted the engine and returned to the airport at slow speed with the flaps extended. An investigation revealed that both wings had suffered structural damage. The shaking was attributed to flutter.

Engineers from the FAA Aircraft Certification Office found evidence that the rear spars of both wings had experienced significant up-and-down oscillations. Bushing holes in the rear spar-attachment fittings were elongated, but the engineers couldn’t conclude whether the elongation existed before the event. If it had, it would have been a major contributing factor to the development of flutter. Also, both the right aileron and the rudder were severely out of balance. Whether control-cable tension was adequate before the flutter event couldn’t be established.

In February 1980, de Havilland issued a service bulletin warning that instances of aileron/wing flutter had been reported and at least two of four conditions must be present to facilitate flutter: 1) the ailerons aren’t balanced; 2) there are slack aileron cables in the wings; 3) the aileron mounting structure isn’t as stiff as it should be; 4) the airplane is being flown outside of limits. Three weeks later, the FAA issued an airworthiness directive requiring inspection of DHC-2 wings, spars and aileron cable tension and balance to eliminate any conditions conducive to flutter.

The NTSB determined that the probable cause of the incident was aerodynamic flutter of the ailerons during normal cruise flight, due to their improper maintenance/balancing, which resulted in structural damage to the airplane’s wings.

It took the NTSB over three years to determine the probable cause of the May 5, 1998, crash of a Mooney M20K in Bakersfield, Calif. The NTSB concluded that the pilot had operated the plane well in excess of its never-exceed speed, which led to elevator flutter and its in-flight breakup. The solo pilot was killed. The IFR flight was from Sacramento to Santa Monica. At 9:03 a.m., the pilot was in contact with Los Angeles Center while cruising at FL190. The pilot asked for and received a course deviation due to a buildup ahead. At 9:23 a.m., the controller approved a descent to 15,000 feet and advised that thunderstorms were moving in from the west. The pilot said that it didn’t look good ahead, and he wanted to head for the Bakersfield Municipal Airport. He was cleared to descend to 14,000 feet, then was handed off to a Bakersfield controller.

At 9:27:20 a.m., the airplane was six miles from the airport, and the Bakersfield controller cleared it to 7,000 feet on a 270-degree heading. At 9:28:21 a.m., the controller radioed the pilot to make a turn “the long way around to a heading of zero niner zero” and handed him off to Bakersfield Approach Control. The pilot said he was descending through 10,200 feet. At 9:29:35 a.m., the pilot was cleared down to 2,300 feet. At 9:30:21 a.m., an ELT signal was received by ATC.

A witness heard an engine at high rpm, followed by a “pop,” then saw the plane exit the overcast cloud layer, followed by debris. The Mooney had undergone an STC conversion to a higher-horsepower engine. The gross weight was increased from 2,900 to 3,200 pounds, but its never-exceed speed stayed at 196 knots. The first components in the wreckage path were the outboard portions of the left and right elevators. Recorded radar data indicated that during cruise flight, the plane’s groundspeed was 191 knots. In the last 24 seconds before radar contact was lost, its groundspeed increased to 240 knots and its average rate of descent was 3,500 fpm.

An FAA specialist reported that “the airplane lost the horizontal tail due to some type of induced flutter, and then tumbled tail up and over until the wings broke off prior to impact with the ground.” The NTSB reported that the onset of elevator flutter in the Mooney M20K occurs at above 241 knots calibrated airspeed.

On November 7, 1997, a twin-engine Cessna 421A was en route from Tomahawk, Wis., to Poplar Grove, Ill. At dusk, the aircraft was climbing through clouds at 700 fpm on autopilot. At 11,000 feet MSL, the ATP-rated pilot heard a loud bang. The autopilot disconnected, he felt a rapid vibration in the control yoke, and the nose pitched down. The pilot slowed, but the vibration continued and pitch trim was ineffective. The pilot advised ATC of the situation, and diverted to Merrill, Wis., where he landed safely.

An FAA examination revealed that the right trim tab and elevator had received extensive damage, and the elevator spar was broken. The rear trim tab linkage bolt was missing. The NTSB determined that the accident’s probable cause was that the elevator trim tab bolt backed out, allowing the trim tab to flutter and destroy the elevator. An improper annual inspection by maintenance personnel and the pilot’s inadequate preflight inspection were also factors.